Mike Tullett wrote:

On Fri, 31 Dec 2004 22:23:19 -0000, Col wrote in


Whereas Co2 actually started to rise rapidly plus the fact it had been
increasing steadily since the beginning of the 20th century So explain
the dip based on the simple equation more Co2 = increased linear warming

The 1940-75 period was not so much of a dip, more of a levelling of
temperature. It was a pause for breath in the general increase in temps
that at been observed since the beginning of the century.
As for it's cause, well probably the natural variability of the climate
held temperatures down during this period.

This particular period was a puzzle for the first IPCC report back in
1991(?). Later work suggested that another form of pollution was a large
factor in this cooling trend - particulates principally from industry when
it was at its dirtiest. Fuels high in sulphur (e.g.coal) also produced
sulphates. The solar reflective qualities of these is thought to have
offset the warming effect of the greenhouse gases during that period.
Once industry began to clean up its act, the latter effect began to
dominate
again. Once the particulates were incorporated into the models, they too
suggested cooling should occur in that period.

Support for this is given by the contrast between the two hemispheres. It
was the dirty Northern Hemisphere which showed the cooling trend. The
southern one only showed a levelling of temperatures as you say.


I don't understand why this 1940-75 period should have been a puzzle for the
IPCC in 1991 since it was explained in 1975. GARP studied paleoclimatic
records covering 700,000 years. The surface temperature data was analysed
in terms of five cycles, ranging from 100,000 to 100 years. The 100-year
cycle peaked around 1940 with a temperature change of -0.0121 K/decade at
1975. The sum of the cycles gave a figure for 1975 of -0.154 K/decade.



Graham